Water-based resin compositon and articles made therefrom

ABSTRACT

An aqueous elastomer dispersion includes a dispersed phase and an aqueous phase. The dispersed phase includes an elastomer including curable aliphatic conjugated-diene elastomers, such as polyisoprene, and a minor amount of at least one additive. The aqueous phase includes water and other optional components in either a soluble state or a dispersion state. The aqueous elastomer dispersion may be prepared by dissolving an elastomer, such as rubber, and additives in a solvent mixture and then converting the resulting solution into an aqueous emulsion. The aqueous emulsion is concentrated and the solvent is stripped from it to yield a dilute latex. The dilute latex that is obtained is concentrated again. Articles made from the aqueous elastomer dispersion include medical gloves, condoms, probe covers, dental dams, finger cots, catheters and the like.

FIELD OF THE INVENTION

The present invention relates to a novel water-based resin compositionthat includes an aqueous elastomer dispersion. Further, the inventionrelates to a method of preparing the aqueous elastomer dispersion andarticles made from the aqueous elastomer dispersion which providesubstantial improvements in performance when compared to those made fromconventionally prepared water-based resin compositions.

BACKGROUND OF THE INVENTION

The use of water-based resin dispersions for making articles such as afilm, coating, and adhesive provides several advantages when compared toits solvent-based counterpart. In particular, water-based dispersionsare environmentally-friendly, easy to formulate, control temperature,and regulate pre-maturation, and have adjustable viscosities.

However, there are also several disadvantages to using a conventionalwater-based resin dispersion. For example, there are limits on both thecompounding of the dispersion and the resulting article. Aftercompounding, a barrier forms between the elastomer dispersion and theremaining components such as dispersions of curing ingredients andadditional polymer dispersions. During the film forming process, theremaining components must overcome this barrier by leaving the confinesof their own dispersion (water, surfactant, and emulsifier), travelingthrough the aqueous medium of the compounding formulation, crossing thesurface of the dispersion of the elastomer particle, and finallypenetrating into the elastomer particles. The likelihood of thisoccurring is even smaller for higher molecular weight components. Thisobstacle also leads to a limitation on the nature of the components thatmay be used.

The second disadvantage is the limitations in the performance of thearticles prepared from a conventional elastomer dispersion. There islimited miscibility between the elastomer and the remaining components,particularly when both have different inherent structures. This causesthe elastomer and the components to remain in their bulk states andinhibits their ability to diffuse and mix in a homogenous manner. As aresult, articles made from conventional elastomer dispersions haveinferior properties.

The third disadvantage is the complexity of formulating conventionalelastomer dispersions and a lack of flexibility in altering theformulations while attempting to improve the performance of articlesmade from the conventional elastomer dispersions.

Thus, there is a need to develop a new elastomer dispersion that canreduce or eliminate these disadvantages. This will result in a dramaticimprovement in the performance of the resulting articles.

SUMMARY OF THE INVENTION

The invention relates to a novel aqueous elastomer dispersion, itsmethod of preparation, and articles made from the aqueous elastomerdispersion. The aqueous elastomer dispersion of the invention includes adispersed phase and an aqueous phase. The dispersed phase includes anelastomer such as polyisoprene and a minor amount of at least oneadditive. The aqueous phase includes water and other optional componentsdissolved in it.

The aqueous elastomer dispersion may be prepared by dissolving anelastomer and additives in a solvent mixture and then converting theresulting solution into an aqueous emulsion. The aqueous emulsion isconcentrated by removing the aqueous phase material and the solvent isstripped from the concentrated emulsion to yield a dilute latex. Thedilute latex that is obtained is preferably concentrated again to adesired solid content.

Articles made from the aqueous elastomer dispersion include medicalgloves, condoms, probe covers, dental dams, finger cots, catheters andthe like.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart illustrating a preferred method for preparing theaqueous elastomer dispersion of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention, which relates to a novel aqueous elastomerdispersion, its method of preparation, and articles made from theaqueous elastomer dispersion formulation, overcomes the obstaclesdiscussed above.

The following terms and definitions are provided to clarify the meaningof specific terms:

A colloidal “dispersion” is an intermediate between a true solution anda mixture, or suspension. It can also be considered an “emulsion,” whichconsists of two liquid phases, a “dispersed phase”, microscopic globs,which are distributed throughout the “dispersing phase”. In oil in waterdispersion (O/W), the dispersing phase is also named as aqueous phase.The emulsion of the present invention is generally referred to as asynthetic colloidal polymer, wherein the polymer may be prepared viaemulsion polymerization (nitrile, polychloroprene), coordination(Ziegler-Natta) polymerization (cis-polyisoprene) or anionicpolymerization (cis-polyisoprene).

“Latex” was originally referred to as a sap from a rubber tree formaking rubber products. Thus, dispersions, emulsions and latex are allconsidered to be kinetically stable, colloidal systems and these termsare used interchangeably for water-based elastomer systems in theinvention.

“Vulcanization” refers to a specific curing process of rubber thatinvolves the addition of sulfur and is normally used in solid rubberprocess. It is an irreversible chemical crosslinking reaction in whichpolymer molecules are linked to other polymer molecules by atomicbridges composed of sulfur atoms.

“Curing” refers to either a chemical reaction that results in a highermolecular weight or a physical process that is associated with asolidified phase change. For the process of the present invention,curing and vulcanization may be used synonymously when referring to aprocess utilizing sulfur or sulfur donors.

A conventional “curing process” of an elastomer is a crosslinkingreaction using curing ingredients including (1) curing/crosslinkingagents such as elemental sulfur and/or various insoluble sulfur donorswhich release sulfur when heated, and/or peroxide curing agents, (2)accelerators which behave like catalysts for the reaction and may becategorized as either primary (e.g., mercaptobenzothiazole MBT andmercaptobenzothiazole disulfide MBTS) or secondary accelerators (e.g.,thiuram, dithiocarbamate and guanidine), and (3) activators such asmetallic oxides and metallic salts (e.g., zinc oxide).

“Creaming” is one of several methods (centrifuging, electro-decantationand evaporation) developed for concentrating newly-made elastomerdispersions. This process can concentrate a low-solid emulsion to, forexample, an above 50% solid emulsion. The creaming process depends onthe difference in specific gravity between water (1.0) and the elastomerpolymer (e.g. about 0.91) or buoyancy of the elastomeric polymer bygathering the particles which are surrounded by serum, near the bottomor near the top of the emulsion. When the separation is complete, theaqueous serum phase is removed, leaving the concentrated emulsion creamphase. Creaming can be accelerated by the addition of certain creamingagents such as sodium alginate, polyvinyl alcohol or cellulosederivatives.

The aqueous elastomer dispersion of the invention includes a dispersedphase and an aqueous phase. The dispersed phase includes an elastomerand a minor amount of at least one additive. The aqueous phase includeswater soluble components which are used in conventional elastomerdispersion formulations, such as surfactants, pH adjusting agents andother water soluble adjuvants. The amount of water soluble components inthe aqueous phase of the elastomer dispersion is typically not more thanabout 10%, and is preferably about 2-10% by weight of total dispersionsolids.

The elastomer may be a curable aliphatic conjugated-diene including, butnot limited to polyisoprene, polychloroprene, styrene-butadienecopolymers, acrylonitrile-butadiene copolymers, styrene blockcopolymers, and butyl rubber. The preferred elastomer of the inventionis polyisoprene, which may be either natural or synthetic. Anysynthetic, linear, high molecular weight polyisoprene with a molecularweight of about 100,000 to about 3,000,000 that is commerciallyavailable can be used.

The at least one additive in the dispersed phase of the aqueouselastomer dispersion may include at least one of curing ingredients,non-curing ingredients, and additional polymers, to be discussed below,with the same, similar or different chemical structures from theelastomer. The total amount of additive(s) used is about 0.5-49% byweight of total dispersion solids.

The curing ingredients may include any such ingredients found inconventional elastomer dispersion compounding formulations. For example,the curing ingredients may include, but are not limited to,sulfur/sulfur donors, accelerators (primary and secondary), andsulfur-curing (or vulcanization) activators and peroxidecuring/crosslinking agents which are known to those skilled in the art.There is no limitation on the chemistry and the number of curingingredients, but they should be capable of dissolving or dispersing inthe solvent system of the elastomeric solution which is described indetail below. The chemical structure of the curing ingredients and theiractivity do not have to be kept intact throughout the emulsificationprocess, but the incorporation of curing ingredient(s) should notadversely affect the desirable performance of the resulting article madeby the dispersion of the present invention.

When curing using sulfur, the main curing agent preferably compriseselemental sulfur (generally believed to be in the form of S8, but not solimited). This may be used alone or in combination with a sulfur donor.A sulfurless system can also be used, but this requires a sulfur donor.Sulfur donors may include, but are not limited to thiuram polysulfidessuch as tetramethylthiuram disulfide and tetraethylthiuram disulfide,which also function as vulcanization accelerators, and xanthogenpolysulfides such as butylxanthogen disulfide, CPB, diisopropylxanthogen polysulfide DIXP, and diisopropyl xanthogen disulfide.

Peroxides may include, but are not limited to, dibenzoyl peroxides suchas one manufactured by R.T. Vanderbilt as Varox A-75, which has a curingtemperature of 120° C./20 minutes for polyisoprene latex, dicumylperoxides such as one manufactured by Akzo Nobel as Perkadox BC-40B,which has a curing temperature of polyisoprene rubber of 120° C./20minutes, and combinations thereof.

Accelerators may include, but are not limited to, dithiocarbamates suchas zinc dimethyl dithiocarbamate (ZDMC), zinc diethyldithiocarbamate(ZDEC), zinc dibutyl dithiocarbamate (ZDBC), zinc dibenzyldithiocarbamate (ZBEC) and zinc pentamethylene dithiocarbamate (ZPD),thiazoles such as 2-mercaptobenzothiazole (MBT), sodium2-mercaptobenzothiazole (SMBT) and zinc 2-mercaptobenzothiazole (ZMBT),thiuram sulfides such as tetramethyl thiuram disulfide (TMTD),tetraethyl thiuram disulfide (TETD and tetrapentamethylene thiuramdisulfide (TPTD), guanidines such as diphenylguanidine (DPG) anddi-o-tolyguanidine (DOTG), and thioureas such as thiourea and diphenylthiourea. One or more accelerators may be used to formulate theelastomer dispersion of the invention.

Activators may include, but are not limited to, zinc oxide, magnesiumoxide and lead oxide. Zinc oxide is the most commonly used vulcanizationactivator. A single accelerator or a synergistic combination ofaccelerators may be used.

Including curing ingredients such as sulfur/sulfur donors, acceleratorsand/or activators in the emulsification process improves the curingmechanism and modifies the topological features of cured articles. Forexample, it provides manufactured articles with such qualities as athinner film, a lower level of chemical residue, and an increased curingefficiency compared with a manufactured article made conventionally byresin dispersion containing the elastomer only and curing ingredientsadded as aqueous dispersions.

Most of the non-curing ingredients used in aqueous elastomer dispersionformulations of the art are solid, moisture sensitive and have a highmolecular weight. This causes the resulting article made from theelastomer dispersion to have such problems as low efficiency ofingredient use, discoloration, property deterioration, and processingdifficulties. These problems can be overcome with the aqueous elastomerdispersion of the invention. In a preparation of the elastomerdispersion of the invention, non-curing ingredients may be included inthe formulation without a series of physiochemical processes such asaggregation, diffusion and migration (which occur with formulation of aconventional elastomer dispersion). Consequently, the performance of theresulting article, such as glove, is improved.

Any non-curing ingredients that are conventionally used in elastomerdispersion compounding formulations may be used in the aqueous elastomerdispersion of the invention. For example, the non-curing ingredients mayinclude, but are not limited to, antioxidants, stabilizers,plasticizers, anti-ozone agents, pigments, fillers, antimicrobialagents, indicators, and additional polymers.

Suitable antioxidants that may be added to the aqueous elastomerdispersion may include, but are not limited to hindered phenols such asbutylated hydroxytoluene (2,6-di-tert-butyl-4-methylphenol) andthiodiethylene bis-di-t-butyl-4-hydroxyphenyl propionate, hinderedpolyphenolics such as butylated reaction products of p-cresol anddicyclopentadiene, hindered phenol/hindered polyphenolics such astrimethyl-tris (di-t-butyl-4-hydroxybenzym)-benzene or octadecyldi-t-butyl-4-hydroxyphenyl propionate, amines such as a blend of 6PPDwith methyl styrene and bis-alpha-dimethylbenzyl diphenyl amine,mixtures such as zinc mercaptotulumimidazole/phenolic, triazinonederivatives such as triazinone-phenol mixtures, polyaromatic amines suchas poly(m-anisidine), phenolic antioxidant hydrazides such as phenolicswith anhydride copolymer, phenolics such as2,2′-methylene-bis-(4-methyl-6-t-butylphenol), cresols such as2,4-dimethyl-6-(1-methylcyclohexyl)-p-cresol, and styrenated phenols. Itis preferred that hindered polyphenolics are used.

Colloidal stabilizers including alkalis for pH adjustment, surfactantsand alkaline caseinates such as sodium caseinate may also be added tothe aqueous phase.

Suitable plasticizers that may be added to the aqueous elastomerdispersion may include, but are not limited to, fatty salts, mineraloils and ester plasticizers.

An antiozonant may be added to the aqueous elastomer dispersion becauseozone severely damages some elastomeric articles, such as highlyunsaturated polyisoprene articles. When included in the aqueouselastomer dispersion of the invention, certain high molecular weightpolymers, such as paraffin wax, EPDM and hydrogenated polydiene canprovide such articles with excellent ozone resistance. Other examples ofantiozonants that may be used in the invention may include, but are notlimited to alkyl/aryl p-phenylenediamines such asN-1,3-dimethylbutyl-N′-phenyl-p-phenylenediamine 6PPD,organoclay-antiozonant complexes such as smectite-containing clay withalkyl-aryl-p-phenylenediamine, functionalized benzotriazoles such asN,N-disubstituted para-phenylenediamine, triazines such as tris(N-1,4-dimethylpentyl-p-phenylenediamino) 1,3,5-triazine and tris(N-alkyl-p-phenylenediamino) 1,3,5-triazine, and p-phenylenediaminessuch as N-isopropyl-N′-phenyl-p-phenylenediamine (IPPD). In addition,polymers including waxes such as paraffinic wax (MW=300-500),microcrystalline wax (MW=600-700) (with paraffinic wax) and low MW PEwax (MW=100-1100), polymeric antiozonants such as polymericdiphenyldiamine, and ozone inert polymers such as EPDM and brominatedisobutylene/para-methylstyrene copolymer (BIMSM) may be used asantiozonants. It is preferred that waxes are used.

Suitable pigments that may be added to the aqueous elastomer dispersionmay include a wide range of natural pigments such as titanium dioxideand iron oxides, and synthetic pigments.

Suitable fillers that may be added to the aqueous elastomer dispersionmay include, but are not limited to, inorganic fillers such as clays,calcium carbonate, talc, and silica and organic fillers such ascrosslinked polymethyl methacrylate, finely divided urethane resinparticles and polyethylene microsphere.

Infection prevention can be effectively achieved when an antimicrobialagent is dispersed in the aqueous elastomer dispersion of the invention.The resulting article, such as an antimicrobial glove, is safer for usebecause the antimicrobial agent does not leach out to any appreciabledegree (e.g. at least 95% is retained in the article after storage atroom temperature and a relative humidity of 60% for three months), as itis intimately mixed/dispersed with the polyisoprene. Examples ofantimicrobial agents that may be used in the polyisoprene dispersion ofthe invention include, but are not limited to, water-insoluble organicphenol compounds such as 5-chloro-2-(2,4-dichlorophenoxy)phenol(triclosan), 2-hydroxy-2,4,6-cycloheptatrien-1-one, o-phenylphenol,2-benzyl-4-chlorophenol, chlorophenol, chlorothymol, andpara-chloro-meta-xylenol (PCMX), inorganic silver compounds such assilver ion-zeolite particle and silver sulfadiazine, zinc compounds suchas zinc pyrithione, copper compounds such as copper oxide, fatty esterssuch as glycerol monocaprate and glycerol monolaurate, silicone-ammoniumpolymers, nitrogen-containing polymers such as polyhexamethylenebiguanide and hydroxybenzoate-containing polymers such aspoly(methacrylate) containing hydroxybenzoate. It is preferred thatorganic phenol compounds are used.

Indicators may also be included in the aqueous elastomer dispersion ofthe invention to provide a signal of status change of the latex article,for example, when a glove is breached. Various indicators are used basedon their physical and chemical properties and function by differentmechanisms. For example, the moisture indicators may include, but arenot limited to, inorganic salts such as cobalt chloride, metal ozonideand ozonide ester, colorants such as triarylmethane, azo dye andalphazurine, phthalein dyes such as thymolsulfonephthalein andphenolphthalein, organic salts such as dyes (phenolphthalein, thymolblue, m-cresol purple) along with transition metals and solvatochromicdyes such as betaine. The microbe indicators may include, but are notlimited to, solvatochromic dyes such as betaine/Reichardt's dye,zwitterion/merocyanine and pyridinium iodide, acid pH dyes such asbromcresol purple and phthalein, and anthocyanin dyes. The bloodindicators may include, but are not limited to, dye intermediates (weakbases) such as O-tolidine OTO and O-dianisidine(3,3′-dimethoxybenzidine), acid-base indicators such as phenolphthalein,and chemiluminescents such as 5-amino-2,3-dihydro-1,4-phthalazinedione(Luminol). Organic salts, solvatochromic dyes and acid-base indicatorsare preferred.

An additional polymer(s) may be used in the dispersed phase of theaqueous elastomer dispersion. Such polymers are preferably selected suchthat when the additional polymer(s) is/are dissolved in the solventsystem, at least part of the polymeric molecule is miscible with theelastomer. The differences between the elastomer and an additionalpolymer may include chemical composition, chain configuration, polymerconformation, molecular weight/molecular weight distribution andcombinations thereof. The additional polymer(s) may be involved in thecuring process or not, but its existence should not compromise finalperformance of the manufactured article. Whether or not polyisoprene isused as the curable aliphatic conjugated-diene elastomer, the additionalpolymer(s) may include, but is/are not limited to, liquid polyisoprene,lower molecular weight polyisoprene, isomers or analogue ofpolyisoprene, a terminal-functionalized derivative of polyisoprene, acopolymer of polyisoprene, polychloroprene, a styrene-butadienecopolymer, acrylonitrile-butadiene copolymers, styrene block copolymers,butyl rubber, and modified forms of these polymers. The aqueouselastomer dispersion of the invention may have a range of 0.5 to 49% ofthe additional polymer in the dispersed phase.

A flowable liquid polyisoprene with a molecular weight of about25,000-50,000 may be used as a processing aid in an elastomerformulation and can be co-dispersed with conventional high molecularweight polyisoprene, as described in the present invention. In additionto its application as barrier sealing, adhesive and elastomeric coating(See U.S. Pat. No. 7,335,807), it can also improve film forming andprovide thin films such as gloves when included in the polyisoprenedispersion of the invention. Examples of non-limiting liquidpolyisoprene materials may include, but are not limited to depolymerizedrubber DPR and synthetic liquid polyisoprene such as Claprene L-IR-30.

Lower molecular weight polyisoprenes function similarly to reactiveplasticizers so they can modulate the curing process. These lowmolecular weight polyisoprenes can make the article thinner, softer andincrease tactile sensitivity. Typically, the molecular weight of suchpolymers is in the range of about 20,000-100,000. They may include butare not limited to Lycopene, Isolene, Synotex 800, and Natsyn 2210.

Structural isomers include cyclized polyisoprene/3,4-polyisoprene,trans-1,4-polyisoprene, and branched polyisoprene/star-shapedelastomers. Cyclized polyisoprene/3,4-polyisoprene, such as Synotex 800and Isogrip, can improve the tear resistance, wet grip, tensile strengthand lower modulus for manufactured articles due to its cyclizedstructure. Trans-1,4-polyisoprene, for example TP-301, provides adifferent chain configuration for film stability against film aging anddimensional changes. Branched and star-shaped polyisoprene, such asShellvis 250, provides an improved fit for articles, such as gloves, anda decreased tackiness due to a unique time-temperature relaxationproperty of its non-linear structure (See Terminal relaxation anddiffusion of entangled three-arm star polymers: Temperature andmolecular weight dependencies, Journal of Polymer Science Part B:Polymer Physics 35 (15) 2503-2510 1997, incorporated herein byreference). Their effectiveness depends on the branching index and avalue greater than 1 is preferred (See Rheological Properties of1,4-Polyisoprene over a Large Molecular Weight Range, Macromolecules, 37(21), 8135-8144, 2004, incorporated herein by reference).

An example of an analogue elastomer is a hydrogenated polyisopreneavailable as LIR 920. It can provide a manufactured article, such as aglove, with a low modulus, low aging degradation, good fit and recoveryupon stretch. This is due to its soft aliphatic chain to providelubrication in the mobility of the polymeric chain when forming a film.

Terminal-functionalized derivatives of the polyisoprene may include, butare not limited to acrylated polyisoprene, carboxylated polyisoprene,epoxidized polyisoprene, sulfonated polyisoprene, and hydroxylatedpolyisoprene. Acrylated polyisoprene may include but are not limited toHEMA- and acrylic acid-modified polyisoprene. They can provide amanufactured article, such as a glove, with an improved chlorinationprocess by way of decreasing the overall chlorination level.Carboxylated polyisoprene can provide improved stability to the aqueouselastomer dispersion of the invention. Additionally, they provide themanufactured article with improved film forming properties. Itshydrophilic nature lends itself to absorbing moisture and providing themanufactured article with breathability. Sulfonated polyisopreneprovides a manufactured article with such properties as antistatic,antimicrobial, improved wet donning, and anti-coagulation properties.

Copolymers of polyisoprene that may be used in the dispersed phase ofthe aqueous elastomer dispersion may include, but are not limited to,di-block and tri-block copolymers. Examples of di-block copolymers mayinclude polyisoprene-b-polybutadiene, polystyrene-b-polyisoprene,polyisoprene-b-polymethyl methacrylate, and polyisoprene-b-polyacrylicacid. Examples of tri-block copolymers may includestyrene-isoprene-styrene (SIS) andstyrene-b-(ethylene-co-butylene)-b-styrene (SEBS). The copolymers may bedissolved in a solvent such as toluene before being incorporated in thedispersed phase. Such copolymers provide the resulting manufacturedarticle with an improved barrier.

A generalized formulation for the vulcanization of water-based latex andthe addition of various additives such as the one shown in Table 1 belowmay be used to prepare the aqueous elastomer dispersion of theinvention. With the exception of the stabilizers, all of the ingredientsin the formulation of Table 1 are in the form of aqueous dispersions(for solids) or aqueous emulsions (for liquids).

TABLE 1 Parts by dry weight rubber (phr) Ingredient Range PreferredRubber latex 100 100 Sulfur and/or Sulfur donors 0.1-5  0.5-2Accelerators 0.1-10  0.5-5 Activators 0.1-5  0.5-2 Antioxidants 0.1-5 0.5-2 Stabilizers as required Plasticizers  0-15  0-5 Antiozonants 0-50.5-2 Pigments 0-5  0-2 Fillers  0-30   0-10 Antimicrobial Agents 0-5 0-1 Indicators 0-5  0-1 Additional Polymers  0-49  0.5-25

The formulation in Table 1 may be used for natural rubber latex or incurrently available commercial synthetic rubber latexes. Generally,using additives as described above in various combinations orconcentrations provides superior properties that are not attainable withcurrently available synthetic latexes. The formulation of Table 1 may bemodified to make the aqueous elastomer dispersion of the invention. Inparticular, for certain applications, a lower concentration of theadditives may be desired to provide better or similar properties.

In one embodiment of the invention, the crosslinking agent, preferablysulfur, is not added into the latex as this may cause prematureexcessive crosslinking of the rubber in the latex particles. In otherembodiments of the invention, the crosslinking agent, preferably sulfurand/or sulfur donors, is/are added as an aqueous dispersion duringcompounding of the aqueous latex before use. Similarly, since thevulcanization activator, preferably zinc oxide, is inorganic, it doesnot dissolve in an organic solvent and cannot be incorporated into thelatex dispersion. Thus, the vulcanization activator may be added as anaqueous dispersion during compounding of the aqueous latex before use.

A method for preparing the aqueous elastomer dispersion from anelastomer such as dry rubber, with additives in the dispersed phaseaccording to the invention is illustrated in the flowchart of FIG. 1.The method is described as follows and in the Examples. The basic stepsoutlined in FIG. 1 are not meant to limit the scope of the invention andmay be repeated or performed in different orders to prepare varioustypes of latexes according to the invention.

As shown in FIG. 1, the starting elastomer, such as rubber, isoptionally processed to reduce its size so that it can easily dissolvein a solvent. A preferred range of sizes for the rubber particles isabout 1-3 mm.

Then, a solution of the elastomer and a minor amount of at least oneadditive in a solvent system is prepared. The solvent system provides amedium for homogenously mixing the elastomer and the additional additiveprior to emulsification. The additives that are to be included in thedispersed phase are dissolved in a solvent or a mixture of solvents. Thesolvents for the additives are not limited, but should be mixable withsolvents that will be used to dissolve the elastomer. Suitable solventsfor additives may include but are not limited to dichloromethane (DCM),chloroform, carbon tetrachloride, acetone, and alcohols.

The concentrations of the accelerators added may be based on theformulation for vulcanizing the rubber provided in Table 1 and areoptimized to provide desired properties of the vulcanized rubber. Asufficient amount of solvent is added to ensure that the additives arestill soluble with the solvent for the rubber. The additives may befirst dissolved in the additive solvent and then combined with therubber solvent or may be dissolved in a mixture of the additive solventand the rubber solvent.

The rubber is dissolved in the solution containing the solvent(s) andthe additives. Alternatively, the rubber may first be dissolved in itsown solvent before adding it to the mixture of additives and theirsolvents. Suitable solvents for elastomers may include but are notlimited to, pentane, hexane, heptane, pentene, toluene, cyclopentane,and cyclohexane. The solvents for the elastomers may be organic liquidsthat have a boiling point of less than 100° C., and preferably less than70° C., preferably at atmospheric pressure or if necessary, with theapplication of a partial vacuum. The solvent system may include a singlesolvent or a solvent mixture including a co-solvent.

The concentration of the rubber in the solvent is governed by thesolubility of the rubber in the solvent. It is preferred to dissolve asmuch rubber as possible in the solution. However, its concentrationshould preferably be no more than 80%, more preferably no more than 60%,and most preferably no more than 50% of the maximum limit of solubilitybecause at maximum solubility, the solution may be too viscous for it tobe mixed with the emulsifier solution and to be broken down into smalldroplets to form an emulsion.

The vessel containing the rubber/additive/solvent mixture may be closedto minimize evaporation and stirred until the rubber is completelydissolved. The temperature of the solvent is preferably maintained atabout 25-35° C., more preferably at about 28-30° C.

The resulting solution containing the rubber, additives, and solvent isthen converted into an aqueous emulsion by a process known to those ofskill in the art, such as those described in U.S. Pat. Nos. 3,250,737,3,285,869, 3,971,746, 3,968,067 and 6,329,444, hereby incorporated byreference. The process may include emulsifying the polymer solutionusing an emulsifier composition, removing the solvent or liquefying thepolymer solution, and combining it with an aqueous medium underconditions that are favorable to stabilizing the emulsion.Alternatively, the water-based dispersion may be formed by following thesteps of emulsifying the polymer solution using an emulsifiercomposition in an aqueous medium and then removing the solvent, asoutlined in FIG. 1.

In general, the emulsifier should be capable of forming a stableemulsion that can withstand the heat during evaporation of the solventand the subsequently formed latex could be destabilized by alkalineearth metal ions, such as calcium ions (e.g., from calcium nitrate usedfor coagulant dipping to make gloves). Emulsifier agents may include,but are not limited to, sodium dodecyl sulfate, sodium dodecyl benzenesulfonate, and carboxylic acid soaps such as caprylic acid soap, capricacid soap, lauric acid soap, oleic acid soap and rosin acid soap ormixture of these. Suitable emulsifiers should preferably have lowfoaming propensities so that excessive foaming does not occur during thevarious process steps of isolating polyisoprene latex concentrate. Theweight of the emulsifier solution is typically about 1-5 wt/vol %,preferably 1-3 wt/vol %, based on the volume of the solvent. Forexample, an emulsifier solution of 2% Nekal BX dry (BASF, 68% sodiumalkyl naphthalene sulphonate) in water adjusted to a pH of 10.5-11.0with, e.g., 3% potassium hydroxide solution may be used.

The latex emulsion is then prepared by pouring therubber/additive/solvent solution into the emulsifier solution which ispreferably stirred, e.g., with a laboratory Silverson L4RT mixer fittedwith a Square Hole High Shear Screen™ at about 3500-4500 rpm. The mixerspeed is varied until the foams that are formed break. In an exemplifiedprocess, after stirring for about 5 minutes at 3500-4500 rpm, the mixerspeed may be increased to about 5000-7000 rpm and stirred for about 10minutes. The mixer speed may then be reduced to about 500-1000 rpm andstirred for about 5 minutes during which time the foams that are formedbreak. If excessive foaming occurs after some time of high speedshearing (5000-7000 rpm), the mixing speed may be reduced to about 1000rpm and stirring continues until the foams break. Then, the mixing speedmay be increased back to 5000-7000 rpm and mixing continues until thetotal time of shearing at this speed is about 10 minutes.

The stirring speed is not limited to the ranges mentioned above, but ischosen so that the final mean diameter of the elastomer particles issimilar to those of commercial latex products. For example, commercialpolyisoprene latex products have a volume mean diameter of about 1micrometer. Typically, a volume mean particle diameter of about 0.5 to1.5 micrometers is desirable. Higher stirring speeds result in a finalelastomer latex particle having a smaller particle diameter. The rangeof speeds can be broadened, depending on the desired application. Forexample, about 5000 rpm-8000 rpm should give the desired final particlesize depending on the total time of high shear mixing. A slightlyshorter time is required when the higher speed is used, and a lowerspeed will result in larger particle size.

Next, the aqueous emulsion is concentrated, whereby at least 20%,preferably at least 30%, and more preferably at least 40% of the aqueousphase is removed resulting in a first emulsion concentrate. The emulsionis preferably concentrated using a creaming agent to increase the rateof separation. The creaming agent may include, but is not limited tosodium alginate, ammonium alginate, methyl cellulose, locust bean gum,gum tragacanth, and carrageenin.

The emulsion containing the creaming agent is stirred at a rate of about1500 rpm for 5 minutes, for example. The mixture may then be filteredthrough a muslin cloth and poured into a separatory funnel. Afterleaving the mixture in the separatory funnel for about 16-20 hours, theemulsion separates into two layers. The upper layer includes aconcentrated emulsion and the lower aqueous layer includes water andpossibly a small amount of emulsion, which is drained off.

Other ways to concentrate the emulsion include using a continuouscentrifugal separator (for example, made by Alpha Laval, Westphalia orSharples), evaporation using a thin film evaporator such as Luwathin-layer evaporator, or ultrafiltration using membranes. On anindustrial scale, the emulsion may be kept in a tall cylindrical tankand at an appropriate time, the lower layer of skim may be drained outleaving an upper layer of concentrated emulsion for further processing.

Once the aqueous layer of the concentrated emulsion is removed, at least90%, preferably at least 95%, more preferably at least 98%, and mostpreferably at least 99% of the solvents are stripped from theconcentrated emulsion to yield a dilute latex. The solvents may bestripped from the concentrated emulsion by, e.g., heating in alaboratory rotary evaporator, such as a Buchi rotary evaporator in whichan evaporating flask is partially immersed in a water bath at atemperature of about 50-55° C. The evaporating flask may be rotated atabout 150 to 200 rpm so that a fresh thin film of the emulsion iscontinuously formed thereby increasing the surface area for evaporationto occur.

The solvent vapor may be condensed and recovered by circulating ethyleneglycol at a temperature of about −15 to −25° C. through the cooling coilof the evaporator and also by immersing the receiving flask of theevaporator in a bath of ethylene glycol at the above temperature.

Typically, a temperature of about 10-15° C. above the boiling point ofthe solvents evaporates off the solvents fairly rapidly. However, toevaporate off traces of residual solvent at the end of the evaporationprocess, the solvents may be heated to a higher temperature of about20-25° C. above their boiling points. The temperature range can bebroadened. For example, if the boiling point of the solvent is high,e.g. about 60-100° C. or greater than 100° C., then it would bedesirable to apply a partial vacuum to the emulsion so that the boilingpoint of the solvent is reduced to less than about 60° C.

As an alternative to using a rotary evaporator to remove the solvent,any suitable device including a thin film evaporator such as a Luwathin-layer evaporator, for example, may be used. On an industrial scale,a large scale rotary evaporator may be used.

After stripping off the solvent, the emulsion containing latex may beconcentrated again to remove any remaining aqueous components bycreaming, e.g., using sodium alginate. For example, about 0.1% sodiumalginate (as a 2% aqueous solution) based on the volume of the emulsionmay be added to the emulsion and mixed well by stirring. The mixture maythen be transferred into a separatory funnel and left overnight forabout 16-18 hours during which time the mixture separates into twolayers. The upper layer contains concentrated latex and the lower layertypically contains a slightly turbid aqueous solution and possibly somelatex. The lower aqueous layer is drained off and a concentrated latexis obtained. Preferably, in this second concentration step, the totalsolids content of the dilute latex is increased by at least three times,preferably at least four times, and more preferably at least five times.For example, the total solids content may increase to about 45-65%.

The concentrated latex may be kept in a separatory funnel for furtherseparation. If this is desired, after a certain amount of time, thelower and predominantly aqueous layer may be drained off again. Thelatex may then be filtered through a muslin cloth. The total solidscontent obtainable from this process is preferably about 40% to about70% by weight, depending on how long the latex is left to separate.Taking into account transportation costs, a higher total solids contentof latex is generally preferred since transporting latex with highertotal solids content means transporting more rubber and less water.

For making gloves, the solids content of compounded latex (i.e. aftervulcanizing ingredients and other additives have been added to thelatex) is about 25% to 40% depending on the thickness of the glovesrequired. Typically, gloves with a higher thickness require higher totalsolids content.

After the latex is concentrated to the desired solids content, the pH ofthe obtained latex may be adjusted, e.g., with an about 5% potassiumhydroxide solution. The pH may be adjusted based on the intendedapplication. For example, the pH range of polyisoprene latex for makinggloves is about 10 to 11.

A method for preparing a polyisoprene latex according to the steps ofFIG. 1 and the description above will be described in detail below inExample 1.

Example 1 Polyisoprene Latex with Additives Wingstay L, ZDEC and DPG

In this example, the additives Wingstay L, an antioxidant (obtained fromGoodyear), and zinc diethyl dithiocarbamate (ZDEC) and diphenylguanidine (DPG), vulcanization accelerators (both obtained fromFlexsys), are first dissolved in solvents. The solutions are prepared bydissolving 0.4 g Wingstay L in 60 mL DCM, dissolving 0.2 g DPG in 10 mLDCM and dissolving 0.1 g ZDEC in 10 mL DCM and then adding these into600 mL of pentane (obtained from Merck) in a 1 liter beaker.

20 g of polyisoprene rubber (Kraton IR) is cut into small pieces andslowly added into a beaker that is continuously stirred and contains theadditive solution which includes 600 mL pentane and 80 mL DCM solventthat contains predissolved 2 phr Wingstay L, 1 phr DPG and 0.5 phr ZDEC.The concentration in Example 1 is about 2.9 weight polyisoprene/volumesolvent. A higher concentration of about 6% weight/volume should befeasible.

The beaker is tightly covered, for example, with a polyethylene sheet tominimize evaporation and the rubber/additive/solvent mixture is stirreduntil the rubber is completely dissolved. The temperature of the solventis maintained at about 25-32° C., more preferably about 28-30° C.

The rubber/additive/solvent mixture is converted to an aqueous emulsionusing a suitable emulsifier and a high shear mixer. An emulsifiersolution of 2% Nekal BX dry (BASF, 68% sodium alkyl naphthalenesulphonate) is prepared be dissolving 18 g Nekal BX dry in 900 mL waterand adjusting the pH to 10.5-11.0 with, e.g., 3% potassium hydroxidesolution. The polyisoprene latex emulsion is then prepared by slowlypouring the rubber/additive/solvent solution into the emulsifiersolution which is stirred, e.g., with a laboratory Silverson L4RT mixerfitted with a Square Hole High Shear Screen™ at about 3500-4500 rpm.

After stirring for about 5 minutes at this speed, the mixer speed isincreased to about 6000-7000 rpm and stirred for about 10 minutes. Themixer speed is then reduced to about 500-1000 rpm and stirred for about5 minutes during which time the foams that are formed break. Then, themixing speed may be increased back to 6000-7000 rpm and mixing continuesuntil the total time of shearing at this speed is about 10 minutes.

The emulsion is concentrated by adding 0.45 g sodium alginate dissolvedin 25 mL water into it and stirring at about 1500 rpm for 5 minutes. Themixture is filtered through a muslin cloth and is then poured into aseparatory funnel. After leaving the mixture in the separatory funnelfor about 16-20 hours, the emulsion separates into 2 layers. The upperlayer includes a concentrated emulsion (about 700-750 mL) and the loweraqueous layer includes water and possibly a small amount of emulsion,which is drained off.

The solvents (in this example, pentane and DCM) are then stripped fromthe concentrated emulsion by, e.g., heating in a laboratory rotaryevaporator, such as a Buchi rotary evaporator in which an evaporatingflask is partially immersed in a water bath at a temperature of about50-55° C. The evaporating flask is rotated at about 150 to 200 rpm sothat a fresh thin film of the emulsion is continuously formed therebyincreasing the surface area for evaporation to occur.

After stripping off the solvent, the emulsion containing polyisoprenelatex is again concentrated by creaming, e.g., using sodium alginate.About 0.1% sodium alginate (as a 2% aqueous solution) based on thevolume of the emulsion is added to the emulsion and mixed well bystirring. The mixture is then transferred into a separatory funnel andleft overnight for about 16-18 hours during which time the mixtureseparated into two layers. The upper layer contains concentrated latexand the lower layer contains a slightly turbid aqueous solution with alittle latex. The lower aqueous layer is drained off and a concentratedlatex is obtained.

The concentrated latex is kept in the separatory funnel for furtherseparation. After about 7 days, the lower and predominantly aqueouslayer is drained off again. The latex is filtered through a muslincloth, and its pH is adjusted to 10.6, e.g., with a 5% potassiumhydroxide solution.

Another example of preparing a polyisoprene latex according to the stepsof FIG. 1 will be described in detail below in Example 2.

Example 2 Polyisoprene Latex with Additives Wingstay L, ZDEC, DPG andMBT

A solution containing four additives is prepared by dissolving 0.44 gWingstay L (obtained from Goodyear, 2 phr) in 60 mL DCM (obtained fromMerck), dissolving 0.22 g DPG (obtained from Flexsys, 1 phr) in 20 mLDCM, dissolving 0.11 g ZDEC (obtained from Flexsys, 0.5 phr) in 20 mLDCM, dissolving 0.11 g MBT (obtained from Merck, 0.5 phr) in 50 mLacetone (obtained from Merck) and then adding the solutions one at atime to 600 mL pentane (obtained from Merck, solvent for polyisoprene)in a 1 liter beaker.

22 g of polyisoprene rubber (Kraton IR) is cut into small pieces andslowly added into the continuously stirred solvent mixture containingthe four predissolved additives described above. The beaker is tightlycovered with a piece of polyethylene sheet to minimize evaporation andthe rubber/additives/pentane/DCM/acetone mixture is stirred until therubber is completely dissolved. The temperature of the solvent mixtureis preferably maintained at about 25-32° C., more preferably about28-30° C.

An emulsifier solution of 2% Nekal BX dry (BASF, 68% sodium alkylnapthalene sulphonate) is prepared be dissolving 18 g of Nekal BX dry in900 mL water and adjusting the pH to 10.5-11.0 with, e.g., a 3%potassium hydroxide solution.

A polyisoprene rubber/additives/solvent emulsion is then prepared byslowly pouring the rubber/additive/solvent solution into the emulsifiersolution which is stirred, e.g., with a laboratory Silverson L4RT mixerfitted with a Square Hole High Shear Screen™ at about 4000-4500 rpm.After stirring for about 5 minutes at this speed, the mixer speed isincreased to about 5800-6000 rpm and stirring continues for about 10minutes. The mixer speed is then reduced to about 500-1000 rpm andstirring continues for about 5 minutes during which time the foams thatare formed breaks. After the foams break, the mixing speed is increasedback to 5800-6000 rpm and mixing continues at this speed for about 10minutes.

The emulsion is concentrated by adding 0.5 g of sodium alginate(obtained from Kimitsu Chemical Industries, Japan) dissolved in 25 mL ofwater and stirring at about 1500 rpm for 5 minutes. The mixture isfiltered through a muslin cloth and then poured into a separatoryfunnel. After leaving the mixture for about 16-20 hours, the emulsionseparates into 2 layers. An upper layer is a concentrated emulsion(about 720 mL) and a lower aqueous layer may contain a small amount ofemulsion, which is drained off.

The solvents (in this case pentane/DCM/acetone) are stripped off fromthe concentrated emulsion by, e.g., heat evaporation in a Buchi rotaryevaporator by heating it in an evaporating flask partially immersed in awater bath at a temperature of about 50-55° C. The evaporating flask isrotated at about 150 to 180 rpm so that a fresh thin film of theemulsion is continuously formed, thereby increasing the surface area forevaporation to occur. When the evaporation slows down (after about 2hours), the temperature of the heating water may be increased to 60-65°C. to complete the evaporation (about 3 hours). The total evaporationtime is typically about 5 hours.

After stripping off the solvent, the emulsion (which is polyisoprenelatex) is concentrated again by creaming, e.g., using sodium alginate.About 0.15% sodium alginate (as a 2% aqueous solution) based on theweight of the emulsion is added to the emulsion and mixed well bystirring. The mixture is then transferred into a separatory funnel andleft for 11 days whereby the mixture separated into two layers. An upperlayer contains concentrated latex and a lower layer contains a slightlyturbid aqueous solution, potentially with a small amount of latex. Thelower aqueous layer is drained off and a concentrated latex is obtained.The latex is filtered through a muslin cloth and its pH adjusted to10.9, e.g., with 5% potassium hydroxide solution.

The aqueous elastomer dispersions of the invention, prepared for exampleas described in Examples 1 and 2, exhibit properties such as solidcontent, particle size, and pH comparable to conventional elastomerdispersions that are commercially available. To demonstrate thisfeature, samples of the polyisoprene latex with additives Wingstay L,DPG and ZDEC, prepared in accordance with Examples 1 and 2, were testedand compared with two different samples of the widely available Kraton®IR401 PI latex products. The data summarized in Table 2 below indicatethat the properties of the aqueous polyisoprene latex of Examples 1 and2, such as volume mean particle diameter, solids content and pH, arecomparable to the Kraton polyisoprene product.

TABLE 2 Aqueous PI Aqueous PI Kraton Kraton Properties Latex, Latex,IR401 Lot IR401 Lot of Latex Example 1 Example 2 B1070119 B1070629Volume Mean 0.45 0.49 0.94 1.04 Particle Diameter (μm) Total Solids 51.652.97 64.8 65.6 Content (%) pH 10.6 10.9 10.7 10.7 Note: Particle sizemeasured using Mastersizer S from Malvern Instruments Ltd.

Additional examples of preparing polyisoprene latex according to thesteps of FIG. 1 described above but having increased concentrations ofaccelerators compared to those of Examples 1 and 2 are described indetail below in Examples 3-5. A latex from Example Control, prepared bya different method, is also tested for comparison with the latexes ofExamples 3-5.

Examples 3-5 And Control Polyisoprene Latex with Varying Amounts ofAdditives Wingstay L, ZDEC, DPG and ZMBT

In Examples 3-5, varying amounts of Wingstay L, ZDEC, and DPG, as shownin Table 3 below, are incorporated with polyisoprene latex into a singledispersion following the method described above in reference to FIG. 1.Example Control has the same formulation as Example 3, as shown in Table3, but is not prepared according to the methods of the presentinvention. Instead, Example Control is prepared according to the methoddescribed in Example 1 of U.S. Pat. No. 6,828,387 (see col. 7, line 45to col. 8, line 67). In U.S. Pat. No. 6,828,387, each of theaccelerators was formulated into a separate dispersion and was addedindividually to a solution containing latex in water. In ExampleControl, accelerators ZDEC, DPG, and ZMBT and Wingstay L that werealready in dispersion form were added to latex in an aqueous solutionand were then mixed together.

The latex particle sizes for Examples 3-5 were increased from the valuesgiven in Example 1 and 2 by stirring the emulsions at a lower speed. ForExamples 3 and 4, the high shear stirring speed was 5000 rpm+/−100 rpmfor 10 minutes instead of 5900 rpm+/−100 rpm for 10 minutes. For Example5, the stirring speed was 5500 rpm+/−100 rpm for 10 minutes. Theparticle sizes for Examples 3-5 are given in Table 3 below.

About 25 g of the latex from each of Examples 3-5 and Control wasweighed in a beaker and stirred with a magnet. Further, S, ZnO, ZMBTwere added as aqueous dispersions and sodium caseinate was added as anaqueous solution to each latex in amounts shown in Table 3 below. ForControl, Wingstay L, ZDEC and DPG were also added as dispersions. Waterwas added to dilute the compounded latex to about 45% total solidscontent and the pH of the compounded latex was adjusted to about 10.5with a dilute potassium hydroxide solution. Each latex mixture was thenfiltered through a muslin cloth. The mixtures were stirred continuouslyfor about 18 hours at an ambient temperature of about 28° C.

Latex films were prepared by casting about 6-8 g of latex on leveledglass plates having an enclosed area of about 60 mm by 100 mm. The latexwas left to dry for about 24 to 48 hours. An anti-tack agent such ascalcium carbonate or starch powder was applied to the dried film and thefilms were stripped off the glass plates. The films were then leached inwater at 70° C. for 1 hour and allowed to dry in the air for 30 minutes.The films were then cured in a hot air oven at 135° C. for about 20minutes. Dumbbell specimens of the cured films were cut for physicaltesting using ASTM Die D and tested according to the ASTM D412-98a testmethod. The results of tests of various physical properties are shownbelow in Table 3.

TABLE 3 Example 3 4 5 Control Ingredients (phr) Wingstay L 2.0 2.0 2.02.0 ZDEC 0.5 0.75 0.75 0.5 DPG 1.0 1.0 1.0 1.0 Sodium caseinate 0.750.75 0.75 0.75 S 1.25 1.25 1.25 1.25 ZnO 0.5 0.5 0.5 0.5 ZMBT 0.5 0.50.69 0.5 Tested Properties Tensile strength (MPa) — 9.4 18.0 21.2 Stressat 500% 0.7 1.3 1.7 Elongation (MPa) Ultimate Elongation (%) 1210 10801022 Mean Volume Diameter (μm) 0.75 0.68 0.64 N/A

The cured film of Example 3 was soft and sticky and could not be tested.As shown in Table 3, increasing the amount of ZDEC incorporated into thelatex particles from 0.5 phr in Example 3 to 0.75 phr in Example 4 gavea cured film with a tensile strength of 9.4 MPa. The film of ExampleControl exhibited higher tensile strengths than that of Example 4.

Increasing the amount of ZDEC incorporated into the latex particles from0.5 phr to 0.75 phr and the amount of ZMBT during compounding from 0.5phr to 0.69 phr in Example 5 resulted in cured films with physicalproperties comparable to that demonstrated by Example Control, which wasprepared using the known method described in Example 1 of U.S. Pat. No.6,828,387. Overall, the results in Table 3 indicate that the novel PIlatex prepared according to the present invention may provide goodphysical properties such as tensile strength. ASTM D3577-06 Type IIclassification standard for surgical gloves requires a minimum tensilestrength of 17 MPa, a maximum stress at 500% elongation of 7.0 MPa, anda minimum ultimate elongation of 650%.

Another approach is to select accelerators that have low solubility inthe aqueous phase, such as those having a longer hydrocarbon chain or ahigher molecular weight. Examples of such accelerators may include, butare not limited to, zinc dibenzyl dithiocarbamate, zinc dinonyldithiocarbamate, zinc di-isononyl dithiocarbamate, and zincN-dodecyl-N-isopropyl dithiocarbamate. Such accelerators have highersolubility in the rubber phase and lower solubility in the aqueousphase.

In U.S. Pat. No. 6,828,387, the amounts of ZDEC, ZMBT and DPG added tothe mixture were in the range of about 0.50 to 1.00 phr. In the presentinvention, the amounts of each of these accelerators may be higher. Inparticular, the amount of ZDEC may be in the range of about 0.2 phr toabout 7.0 phr, preferably about 0.5 phr to about 3.0 phr, the amount ofDPG may be in the range of about 0.2 to 10.0 phr, preferably about 0.5phr to about 4.0 phr, and the amount of ZMBT may be in the range ofabout 0.2 phr to about 10.0 phr, preferably about 0.5 phr to about 4.0phr.

In the latex made according to the process of the present invention, theadditives are already incorporated into the elastomer beforecompounding. This is because the additives are dissolved in a suitablesolvent, combined with the polyisoprene solution, and then made into adispersion. This method differs from other known methods, such as themethod recited in Example 1 of U.S. Pat. No. 6,828,387 and in ExampleControl above, where separate dispersions/emulsions of each of thecomponents (elastomer and additives) are obtained from suppliers and arethen combined together.

Dispersions/emulsions are, by nature, thermodynamically unstablesystems, and in a multiple formulation process, there are manychallenges to obtaining a desirable uniform mixture. In particular, itis difficult to match the stabilization of each of the individualdispersions/emulsion systems, and because of this, a broad particle sizedistribution often results. The method of the present invention mayovercome this disadvantage.

Further, the process for converting the solution containing these threeadditives into latex is the same as that for converting a solution ofpolyisoprene without the additives into latex. Thus, this method doesnot require extra energy. This process allows for a savings in energy,equipment cost, materials (a lower amount of surfactant is needed tostabilize the dispersion), and labor required for dispersing these threeingredients when compared to the method recited in Example 1 of U.S.Pat. No. 6,828,387.

In addition to Wingstay L, ZDEC and DPG, copolymers of polyisoprene maybe incorporated into the dispersed phase of the polyisoprene latex bythe method described below in Examples 6 and 7, which follow the methodoutlined in FIG. 1.

Example 6 Polyisoprene Latex with Additives Wingstay L, ZDEC, DPG andSIS (Quintac 3421)

The copolymer of polyisoprene used in this example was astyrene-isoprene-styrene (SIS) diblock copolymer obtained from ZeonChemicals under the trade name Quintac 3421. Quintac 3421 has a styrenecontent of 14%. A solution was prepared by dissolving 0.44 g Wingstay L(obtained from Goodyear, 2 phr) in 60 mL DCM (obtained from Merck),dissolving 0.22 g DPG (obtained from Flexsys, 1 phr) in 20 mL DCM,dissolving 0.165 g ZDEC (obtained from Flexsys, 0.75 phr) in 20 mL DCM,and dissolving 3.3 g Quintac 3421 (obtained from Zeon Chemicals, 15 phr)in 30 mL toluene (obtained from Merck). The solutions were then addedone at a time to 600 mL pentane (obtained from Merck, solvent forpolyisoprene) in a 1 liter beaker. The solution of Quintac 3421 intoluene was very viscous and 3 mL of toluene was used for rinsing theresidue sticking on the glass container.

g of polyisoprene rubber (Kraton IR) was cut into small pieces andslowly added into the continuously stirred solvent mixture containingthe four predissolved additives described above. The beaker was tightlycovered with a piece of polyethylene sheet and tied with a rubber bandto minimize evaporation and the rubber/additives/pentane/DCM/toluenemixture was magnetically stirred for about 3 hours whereby the rubberwas completely dissolved. The temperature of the solvent mixture wasmaintained at about 28-30° C.

An emulsifier solution of 2% Nekal BX dry (BASF, 68% sodium alkylnapthalene sulphonate) was prepared be dissolving 18 g of Nekal BX dryin 900 mL water and adjusting the pH to 10.5-11.0 with a few drops of 3%potassium hydroxide solution.

A polyisoprene rubber/additives/solvent emulsion was then prepared byslowly pouring the rubber/additive/solvent solution into the emulsifiersolution which was stirred with a laboratory Silverson L4RT mixer fittedwith a Square Hole High Shear Screen™ at 4400-4600 rpm. After stirringfor 5 minutes at this speed, the mixer speed was increased to 4900-5100rpm and stirring continued for about 10 minutes. The mixer speed wasthen reduced to about 1000 rpm and stirring continued for about 5minutes during which time the foams that were formed break. No excessivefoaming was noted under these conditions.

The emulsion was concentrated by adding 0.5 g of sodium alginate(obtained from Kimitsu Chemical Industries, Japan) dissolved in 25 mL ofwater and stirring at about 1500 rpm for 5 minutes. The mixture wasfiltered through a muslin cloth and then poured into a separatoryfunnel. After leaving the mixture for about 16-20 hours, the emulsionseparated into 2 layers. An upper layer was a concentrated emulsion(about 810 mL) and a lower aqueous layer contained a small amount ofemulsion, which was drained off.

The organic solvents pentane/DCM/toluene were stripped off from theconcentrated emulsion by heat evaporation in a Buchi rotary evaporatorby heating the emulsion in an evaporating flask partially immersed in awater bath at a temperature of about 45° C. for 3 hours followed by 50°C. for 4 hours. The evaporating flask was rotated at about 210 rpm sothat a fresh thin film of the emulsion was continuously formed, therebyincreasing the surface area for evaporation to occur. The solvent vaporwas condensed and recovered by circulating ethylene glycol at atemperature of about −15° C. to −25° C. through the cooling coil of theevaporator and also immersing the receiving flask of the evaporator in abath of ethylene glycol at the above temperature. After that, toluenewhich has a higher boiling point, was evaporated at about 60° C. underreduced pressure of about 100 torr for 4 hours. The weight of theconcentrated latex was 246.2 g.

The latex was further concentrated by creaming using sodium alginate.0.25 g sodium alginate dissolved in 15 mL water was added to the latexand mixed well by stirring. The mixture was then transferred into aseparatory funnel and left for 10 days during which the mixtureseparated into two layers. An upper layer contained concentrated latexand a lower layer contained a slightly turbid aqueous solution with alittle latex. The lower aqueous layer was drained off and a concentratedlatex was obtained. The latex was filtered through a muslin cloth andits pH was adjusted to 10.95 with 5% potassium hydroxide solution. Theweight of concentrated latex obtained was 42.16 g with a total solidscontent of 55.4%. The mean volume diameter of the particles was 0.69micrometers.

This latex was compounded with 0.75 phr sodium caseinate added asaqueous solution, 1.25 phr sulfur added as dispersion, 0.5 phr zincoxide added as dispersion, and 0.69 phr ZMBT added as dispersion. Waterwas added to dilute the mixture to a total solids content of about 45%and the pH of the compounded latex was adjusted to about 10.5 with adilute potassium hydroxide solution. The latex was filtered through amuslin cloth into a beaker and was then covered and stirred with amagnet for 20 hours at ambient temperature (26-28° C.). Latex films wereprepared by casting 7 to 8 g of latex on leveled glass plates having anenclosed area of 60 mm by 100 mm. The latex was left to dry for about 48hours. Calcium carbonate powder was applied on the surfaces of the driedfilms and the films were stripped off the glass plates. The films werethen leached in water at 70° C. for 1 hour and were hung to dry in theair for 30 minutes. They were then cured in hot air oven at 135° C. for20 minutes.

Dumbbell specimens of the cured films were cut for physical testingusing ASTM Die D and tested according to ASTM D412-98a method as shownin Table 4 below.

Example 7 Polyisoprene Latex with Additives Wingstay L, ZDEC, DPG andSIS (Vector 4111A)

The copolymer of polyisoprene used in this example was an SIS triblockcopolymer with the trade name Vector 4111A (obtained from DexcoPolymers) that has a styrene content of 18%. A solution was prepared bydissolving 0.44 g Wingstay L (obtained from Goodyear, 2 phr) in 60 mLDCM (obtained from Merck), dissolving 0.22 g DPG (obtained from Flexsys,1 phr) in 20 mL DCM, dissolving 0.165 g ZDEC (obtained from Flexsys,0.75 phr) in 20 mL DCM, dissolving 3.3 g Vector 4111A (obtained fromDexco Polymers, 15 phr) in 30 mL toluene (obtained from Merck) and thenadding the solutions one at a time to 600 mL pentane (obtained fromMerck, solvent for polyisoprene) in a 1 liter beaker. The solution ofVector 4111A in toluene was very viscous and 3 mL of toluene was usedfor rinsing the residue sticking on the glass container.

22 g of polyisoprene rubber (Kraton IR) was cut into small pieces andslowly added into the continuously stirred solvent mixture containingthe four predissolved additives described above. The beaker was coveredtightly with a piece of polyethylene sheet and tied with a rubber bandto minimize evaporation and the rubber/additives/pentane/DCM/toluenemixture was magnetically stirred for about 3 hours whereby the rubberwas completely dissolved. The temperature of the solvent mixture wasmaintained at about 28-30° C.

An emulsifier solution of 2% Nekal BX dry (BASF, 68% sodium alkylnapthalene sulphonate) was prepared by dissolving 18 g of Nekal BX dryin 900 mL water and adjusting the pH to 10.5-11.0 with a few drops of 3%potassium hydroxide solution.

A polyisoprene rubber/additives/solvent emulsion was then prepared byslowly pouring the rubber/additive/solvent solution into the emulsifiersolution which was stirred with a laboratory Silverson L4RT mixer fittedwith a Square Hole High Shear Screen™ at 4400-4600 rpm. After stirringfor 5 minutes at this speed, the mixer speed was increased to 4900-5100rpm and stirring continued for about 10 minutes. The mixer speed wasthen reduced to about 1000 rpm and stirring continued for about 5minutes during which time the foams that were formed break. No excessivefoaming was noted under these conditions.

The emulsion was concentrated by adding 0.5 g of sodium alginate(obtained from Kimitsu Chemical Industries, Japan) dissolved in 25 mL ofwater and stirring at about 1500 rpm for 5 minutes. The mixture wasfiltered through a muslin cloth and then poured into a separatoryfunnel. After leaving the mixture for about 16-20 hours, the emulsionseparated into 2 layers. An upper layer was a concentrated emulsion(about 800 mL) and a lower aqueous layer contained a small amount ofemulsion, which was drained off.

The organic solvents pentane/DCM/toluene were stripped off from theconcentrated emulsion by heat evaporation in a Buchi rotary evaporatorby heating the emulsion in an evaporating flask partially immersed in awater bath at a temperature of about 45° C. for 3 hours followed by 50°C. for 4 hours. The evaporating flask was rotated at about 210 rpm sothat a fresh thin film of the emulsion was continuously formed, therebyincreasing the surface area for evaporation to occur. The solvent vaporwas condensed and recovered by circulating ethylene glycol at atemperature of about −15° C. to −25° C. through the cooling coil of theevaporator and also immersing the receiving flask of the evaporator in abath of ethylene glycol at the above temperature. After that, toluene,which has a higher boiling point, was evaporated at about 60° C. underreduced pressure of about 100 torr for 4 hours. The weight of theconcentrated latex was 241.0 g.

The latex was further concentrated by creaming using sodium alginate.0.25 g sodium alginate dissolved in 15 mL water was added to the latexand mixed well by stirring. The mixture was then transferred into aseparatory funnel and left for 10 days whereby the mixture separatedinto two layers. An upper layer contained concentrated latex and a lowerlayer contained a slightly turbid aqueous solution with some latex. Thelower aqueous layer was drained off and a concentrated latex wasobtained. The latex was filtered through a muslin cloth and its pH wasadjusted to 10.99 with a 5% potassium hydroxide solution. The weight ofconcentrated latex obtained was 41.52 g with a total solids content of54.27%. The mean volume diameter was 0.57 micrometers.

25 g of the latex was compounded with 0.75 phr sodium caseinate added asaqueous solution, 1.35 phr sulfur added as dispersion, 0.60 phr zincoxide added as dispersion, and 0.75 phr ZMBT added as dispersion. Waterwas added to dilute the mixture to a total solids content of about 45%,and the pH of the compounded latex was adjusted to about 10.5 with adilute potassium hydroxide solution. The latex was then filtered througha muslin cloth into a beaker and was covered and stirred with a magnetfor 40 hours at ambient temperature (about 26-28° C.). In this example,the latex was stirred for 40 hours instead of 20 hours as in Example 6.

The compound was converted to a cured film, as described in Example 6above. Dumbbell specimens of the cured films were cut for physicaltesting using ASTM Die D and tested according to ASTM D412-98a method.

Physical properties of the cured latex films of Examples 6 and 7 weretested and compared with the ASTM D3578-05 Standard Specification forRubber Examination Gloves, Type II classification. The results aresummarized in Table 4.

TABLE 4 Tensile Stress at 500% Ultimate strength elongation, M500elongation (MPa) (MPa) (%) Example 6 15.2 1.5 1070 Example 7 16.4 1.41090 ASTM D3578-05, 14.0 (minimum) 2.8 (maximum) 650 (minimum) Type II

As indicated in Table 4, the cured polyisoprene-SIS latex films ofExamples 6 and 7 met the ASTM D3578-05 standards. Moreover, the stress(or modulus) at 500% elongation, M500, were surprisingly low andcomparable to that of the cured polyisoprene films discussed above (seeExample Control above which has M500 of 1.7 MPa). A low modulus is adesirable property because the resulting product feels soft andcomfortable and provides good tactile sensitivity to the user. The M500values of the films of Examples 6 and 7 were significantly lower thanthat of natural rubber gloves which may have an M500 value of about 3.9MPa (range 2.6-5.0 MPa). Polyisoprene gloves typically have M500 valuesof about 2.0 MPa (range 1.4-2.5 MPa).

The method of the present invention may incorporate up to 20%,preferably up to 30%, more preferably up to 40%, and most preferably upto 49% (by weight of total dispersion solids) of SIS to the dispersedphase. Typically, SIS cannot be milled down to form dispersions such asthe ones used for sulfur, vulcanization accelerators, antioxidants, etc.because it is not a hard solid and a commercial SIS latex is notavailable. Thus, SIS cannot be blended with polyisoprene latex and madeinto a product using conventional methods.

An advantage of using SIS in combination with elastomers such aspolyisoprene is that SIS is significantly less expensive thanpolyisoprene and the amount of polyisoprene used in the latex may bereduced by incorporating SIS, thus reducing the cost of the rawmaterials. By incorporating SIS in the dispersed phase, improved tearstrength, puncture strength, solvent resistance, and degradationresistance properties, as well as processing advantages may be obtained.

The method described in Examples 6 and 7 is not limited to the additionof SIS to elastomers such as polyisoprene. As noted above, otherpolymers may also be incorporated with the elastomer using the processof the invention. In addition, SIS may be combined with the elastomerwithout additives. An elastomer-SIS latex without such additives maythen be converted to a cured film by using the methods discussed above.

Advantages of the latex of the invention, process of making the latex,and articles manufactured from the latex include improved film forming(less surfactant/emulsifier compared to conventional composition),performance modification of gloves and other articles (protection ofsensitive additives), and enhanced formulation properties (formulationstability and compatibility). In addition, the latex provides improvedbulk properties of gloves and other articles (softness, barrier andoxidative degradation resistance), reduced surface contamination, e.g.cleaner glove (less impurity migration to surface), increasedformulation efficiency (availability of additive without diffusionloss), improved maturation process (no diffusion process ofingredients/additives), and reduced manufacturing costs (less individualadditive production).

The aqueous polyisoprene latex of the invention demonstrates benefitsover the Kraton product because elastomeric articles made from theinventive aqueous polyisoprene latex provide substantial improvements inperformance in the various applications since a wider range of additivesmay be included in the latex. The improvements in the performance dependon the nature and amount of the additives dissolved in polyisoprenesolution prior to emulsification. For example, the advantages of anarticle such as a glove made from the aqueous elastomer dispersion ofthe invention include mechanical properties such as barrier performance,touch-feel sensation, tactile sensitivity, tear resistance, andinstrument gripping, and enhanced glove surface performances such asantimicrobial activity, antistatic property, a clean glove surface, andsweat absorption. Thus, according to the invention, additives may beselected to improve specific properties of products in a wider range ofapplications than previously possible.

The aqueous water-based elastomer composition of the invention may beprocessed to make a variety of articles including, but not limited tomedical gloves, condoms, probe covers (i.e., for ultrasonic andtransducer probes), dental dams, finger cots, catheters and the like.Methods for processing the composition may include, but are not limitedto coagulant dipping, dispersion dipping, drying, leaching and ovencuring.

1. A method of preparing an aqueous elastomer dispersion, comprising thesteps of: preparing an elastomer/additive solution comprising anelastomer, at least one additive, and a solvent system; emulsifying theelastomer/additive solution to form an emulsion; concentrating theemulsion to form a concentrated emulsion; removing substantially all ofthe solvent system from the concentrated emulsion to form asubstantially solvent-free dispersion; and optionally concentrating thesubstantially solvent-free dispersion.
 2. The method of claim 1, whereinthe elastomer is selected from the group consisting of polyisoprene,polychloroprene, styrene-butadiene copolymers, acrylonitrile-butadienecopolymers, styrene block copolymers, and butyl rubber.
 3. The method ofclaim 2, wherein the elastomer has a molecular weight of about 100,000to about 3,000,000.
 4. The method of claim 2, wherein the at least oneadditive is selected from the group consisting of sulfur/sulfur donors,peroxide curing/crosslinking agents, vulcanization accelerators,vulcanization activators, antioxidants, stabilizers, plasticizers,antiozonants, pigments, fillers, antimicrobial agents, indicators, andadditional polymers.
 5. The method of claim 4, wherein an amount of theat least one additive is about 0.5-49% by weight of total solids of theaqueous elastomer dispersion.
 6. The method of claim 1, wherein thesolvent system includes a first solvent and second solvent.
 7. Themethod of claim 6, wherein the elastomer is soluble in the firstsolvent, and the at least one additive is soluble in the second solvent.8. The method of claim 6, wherein the first solvent includes one or moresolvents, and the second solvent includes one or more solvents.
 9. Themethod of claim 1, wherein the emulsifying step comprises adding anemulsifier.
 10. The method of claim 9, wherein the emulsifying stepfurther comprises mixing with a high shear mixer.
 11. The method ofclaim 1, wherein one or both of the concentrating steps comprise addinga creaming agent.
 12. The method of claim 11, wherein one or both of theconcentrating steps further comprise separating a concentrate and anaqueous solution.
 13. The method of claim 1, wherein one or both of theconcentrating steps is carried out by centrifuging using a continuouscentrifugal separator.
 14. The method of claim 1, wherein the removingstep comprises evaporating the solvent system from the concentratedsolution.
 15. The method of claim 1, wherein the removing step comprisesheating the concentrated solution to evaporate the solvent system. 16.The method of claim 1, further comprising the step of adjusting the pHof the aqueous elastomer dispersion.
 17. The method of claim 1, whereinthe total solids content of the aqueous elastomer dispersion is about45-65% by weight.
 18. The method of claim 1, further comprising the stepof making an article from the aqueous elastomer dispersion.
 19. Themethod of claim 18, wherein the article is selected from the groupconsisting of gloves, condoms, probe covers, dental dams, finger cots,and catheters.